Create large ridges or ledges (>50 mm) on intertidal artificial structures

Overall effectiveness category Awaiting assessment
Number of studies: 3
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Background information and definitions

Definition: ‘Large ridges and ledges’ are elevations with a length to width ratio >3:1 that protrude >50 mm from the substratum (modified from “Large elevations” in Strain et al. 2018). On vertical surfaces, vertically-orientated elevations that fit these criteria are referred to as ‘ridges’, while horizontal ones are referred to as ‘ledges’. On horizontal surfaces, these features are referred to as ‘ridges’ regardless of their orientation.

Large ridges and ledges create vertical or horizontal (i.e. overhangs) relief in intertidal rocky habitats. They can provide organisms refuge from desiccation and temperature fluctuations during low tide (Williams & Morrit 1995) and alter flow velocities (Guichard & Bourget 1998). Some species preferentially recruit to habitats with high vertical or horizontal relief, potentially to avoid predators (Harmelin-Vivien et al. 1995). The size and density of ridges and ledges is likely to affect the size, abundance and variety of organisms that can use them. Small habitats can provide refuge for small-bodied organisms but may exclude larger organisms and limit their growth. Large habitats can be used by larger-bodied organisms but may not provide sufficient refuge from predators for smaller organisms. By default, horizontal ledges (overhangs) create shaded and downward-facing surfaces, which can be associated with the presence of non-native species (Dafforn 2017).

Ridges and ledges are sometimes present on quarried boulders used in marine artificial structures (MacArthur et al. 2020) but are often absent from other types of structures. Large ridges and ledges can be created on intertidal artificial structures by adding material, either during construction or retrospectively.

^{Dafforn K.A. (2017) Eco-engineering and management strategies for marine infrastructures to reduce establishment and dispersal of non-indigenous species. Management of Biological Invasions, 8, 153–161.}

^{Guichard F. & Bourget E. (1998) Topgraphic heterogeneity, hydrodynamics, and benthic community structure: a scale-dependence cascade. Marine Ecology Progress Series, 171, 59–70.}

^{Harmelin-Vivien M.L., Harmelin J.G. & Leboulleux V. (1995) Microhabitat requirements for settlement of juvenile sparid fishes on Mediterranean rocky shores. Hydrobiologia, 300, 309–320.}

^{MacArthur M., Naylor L.A., Hansom J.D. & Burrows M.T. (2020) Ecological enhancement of coastal engineering structures: passive enhancement techniques. Science of the Total Environment, 740, 139981.}

^{Strain E.M.A., Olabarria C., Mayer-Pinto M., Cumbo V., Morris R.L., Bugnot A.B., Dafforn K.A., Heery E., Firth L.B., Brooks P.R. & Bishop M.J. (2018) Eco-engineering urban infrastructure for marine and coastal biodiversity: which interventions have the greatest ecological benefit? Journal of Applied Ecology, 55, 426–441.}

^{Williams G.A. & Morritt D. (1995) Habitat partitioning and thermal tolerance in a tropical limpet, Cellana grata. Marine Ecology Progress Series, 124, 89–103.}

Study locations

Key messages

Three studies examined the effects of creating large ridges or ledges on intertidal artificial structures on the biodiversity of those structures. Two studies were in an estuarine sound in northwest USA and one was on an open coastline in the UK.

COMMUNITY RESPONSE (2 STUDIES)

Overall richness/diversity (2 studies): One of two replicated, controlled studies (including one randomized study) in the USA and the UK reported that creating large ledges on intertidal artificial structures, along with grooves and small protrusions, increased the combined macroalgae, microalgae and invertebrate species diversity on structure surfaces. One study found that creating large ridges, along with large protrusions, did not increase the combined macroalgae and invertebrate species richness.

POPULATION RESPONSE (3 STUDIES)

Overall abundance (1 study): One replicated, randomized, controlled study in the USA reported that creating large ledges on intertidal artificial structures, along with grooves and small protrusions, increased the combined macroalgae, microalgae and invertebrate abundance on structure surfaces.
Algal abundance (1 study): One replicated, randomized, controlled study in the USA found that creating large ledges on intertidal artificial structures, along with grooves and small protrusions, increased the rockweed abundance on structure surfaces.
Invertebrate abundance (2 studies): Two replicated, controlled studies (including one randomized study) in the USA and the UK found that creating large ledges or ridges on intertidal artificial structures, along with grooves and small protrusions, or large protrusions, increased the abundance of mussels or limpets, but not barnacles, on structure surfaces.
Fish abundance (1 study): One before-and-after study in the USA reported that creating large ledges on an intertidal artificial structure, along with grooves and small protrusions, did not increase juvenile salmon abundance around the structure.

BEHAVIOUR (1 STUDY)

Fish behaviour change (1 study): One before-and-after study in the USA reported that creating large ledges on an intertidal artificial structure, along with grooves and small protrusions, increased juvenile salmon feeding activity around the wall.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A replicated, randomized, controlled study in 2008–2011 on three intertidal seawalls in Puget Sound estuary, USA (Cordell et al. 2017) reported that large ledges created on seawall panels, along with grooves and small protrusions, supported higher macroalgae, microalgae and invertebrate species diversity and live cover, with more rockweed Fucus distichus and mussels Mytilus spp., than seawall surfaces without added habitats. After 42 months, the macroalgae, microalgae and invertebrate species diversity was higher on seawall panels with ledges, grooves and protrusions than on seawall surfaces without (data reported as Evenness index, not statistically tested). Total live cover was 83–84% on panels with ledges, grooves and protrusions and 74% on surfaces without (data not statistically tested). Rockweed and mussel abundances were statistically similar on panels with long ledges (rockweed: 5% cover; mussels: 6%) and short ledges (rockweed: 13%; mussels: 12%), and higher on both than on seawall surfaces without (both 1%). Abundances of six other species groups were not statistically tested (see paper for results). It is not clear whether these effects were the direct result of creating ledges, grooves or protrusions. Large ledges were created on concrete seawall panels (height: 2.3 m; width: 1.5 m; thickness: ~150 mm) using a formliner. Each panel had three long (length: ~1.5 m; width/height: ~0.5 m) or six short (length: ~0.7 m; width: ~0.2 m; height: ~0.5 m) evenly-spaced horizontal ledges. Panels were either smooth or had grooves and small protrusions on their surfaces. One panel of each ledge-surface combination was randomly arranged spanning high–lowshore on each of three vertical concrete seawalls in January 2008. Seawall surfaces were intertidal areas of seawall cleared of organisms (dimensions/spacing not reported). Macroalgae, microalgae and invertebrates were counted on panels (excluding downward-facing surfaces) and seawall surfaces during low tide after 42 months.
Study and other actions tested
Referenced paper
Cordell J.R., Toft J.D., Munsch S. & Goff M. (2017) Benches, beaches, and bumps: how habitat monitoring and experimental science can inform urban seawall design. Pages 421-438 in: D.M. Bilkovic, M.M. Mitchell, P.M.K. La & J.D. Toft (eds.) Living Shorelines: The Science And Management Of Nature-Based Coastal Protection. CRC Press, Boca Raton, Florida.
A replicated, controlled study in 2015–2017 on an intertidal seawall on open coastline in the UK (MacArthur et al. 2020) found that boulders positioned with large ridges on their upper surfaces, along with large protrusions, supported similar macroalgae and invertebrate species richness and barnacle Semibalanus balanoides abundance, but higher limpet Patella vulgata abundance, than boulders positioned randomly. Boulders positioned with large ridges and protrusions on their upper surfaces supported similar macroalgae and invertebrate species richness (4 species/boulder) and barnacle abundance (data not reported) but more limpets (82 limpets/boulder) than boulders positioned randomly (4 species/boulder, 27 limpets/boulder). It is not clear whether these effects were the direct result of creating large ridges or protrusions. Ten granite boulders (width: 2 m) were intentionally positioned with naturally-occurring large ridges and/or protrusions on their upper surfaces (average 4/boulder) and ten were positioned randomly (1/boulder) at mid-highshore in a granite boulder seawall during construction in 2015–2017. Ridges/protrusions were 100–800 mm high (other dimensions/spacing not reported). Macroalgae and invertebrates on the upper surfaces of boulders were counted during low tide in June 2017.
Study and other actions tested
Referenced paper
MacArthur M., Naylor L.A., Hansom J.D. & Burrows M.T. (2020) Ecological enhancement of coastal engineering structures: passive enhancement techniques. Science of the Total Environment, 740, 139981.
A before-and-after study in 2012–2018 on an intertidal seawall in Puget Sound estuary, USA (Sawyer et al. 2020) reported that creating large ledges on the seawall, along with grooves and small protrusions, did not increase juvenile salmon Oncorhynchus spp. abundance around the wall but increased their feeding activity. Data were not statistically tested. Juvenile salmon abundances were lower after large ledges were created during seawall reconstruction (5–151 individuals/m²) compared with before (47–431/100m²), but the frequency of their feeding behaviour increased by 6–27%. It is not clear whether these effects were the direct result of creating ledges, grooves and protrusions, increased light levels or reduced water depth in front of the wall. Large ledges (length: 2 m; width: 0.6 m; height: 0.2 m) were created on concrete seawall panels using a formliner. Each panel had one horizontal ledge at high, mid or lowshore and grooves and small protrusions on their surfaces. Panels were attached to a vertical concrete seawall during reconstruction in 2017 (numbers/month not reported). Light-penetrating panels were also installed to increase light around the wall, and the seabed was raised in front. Juvenile salmon within 10 m of the wall were surveyed from 20–minute snorkels at high and low tide during March–August at three sites along the wall before reconstruction in 2012 (35 surveys), and at three different sites along the wall after reconstruction in 2018 (42 surveys).
Study and other actions tested
Referenced paper
Sawyer A.C., Toft J.D. & Cordell J.R. (2020) Seawall as salmon habitat: eco-engineering improves the distribution and foraging of juvenile Pacific salmon. Ecological Engineering, 151, 105856.

Please cite as:

Evans, A.J., Moore, P.J., Firth, L.B., Smith, R.K., and Sutherland, W.J. (2021) Enhancing the Biodiversity of Marine Artificial Structures: Global Evidence for the Effects of Interventions. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

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This Action forms part of the Action Synopsis:

Biodiversity of Marine Artificial Structures

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